This invention relates to a multilayer substrate including glass-ceramic insulator layers.
A multilayer ceramic substrate is for use in mounting a plurality of semiconductor chips directly thereon. Each semiconductor chip may be an IC or an LSI. The substrate comprises a plurality of ceramic layers as insulator layers and, between two adjacent insulator layers, a plurality of wirings for the semiconductor chips. The wirings are referred to also as circuit or conductor patterns.
Multilayer ceramic substrates are manufactured either by (A) a thick film printing method or (B) a green sheet method. The green sheet method is carried out either by (a) a laminating technique or (b) a printing technique.
The thick film printing method is widely put into practice. According to the thick film printing method, conductor layers for the wirings and insulator layers are alternatingly printed on a ceramic base. Each of the conductor and the insulator layers is fired or sintered after having been printed. The thick film printing method is therefore time consuming. It is difficult to manufacture an excellent substrate by reducing the dimensions of through or via holes formed through the insulator layers for the wirings, by rendering the wirings fine, and by stacking a desired number of insulator layers. In addition, it is difficult to attain a high yield of manufacture and a high reproducibility of the substrates.
According to the green sheet laminating technique, green sheets are successively laminated and then sintered or fired into insulator layers with the wirings printed on each green sheet before the successive laminating. In a conventional green sheet laminating technique, alumina is used as the insulating material. The laminated green sheets must therefore be sintered at as high a sintering or firing temperature as 1,500.degree. C. or higher. In view of the high sintering temperature, the wirings must be made of molybdenum, tungsten, or a like conductor material having a high melting point.
For such a high melting point conductor material, sintering must be carried out in a reducing atmosphere as, for example, in a hydrogen furnace. Due to the necessity of a high sintering temperature and a reducing atmosphere, the installation for manufacture is large scale, expensive, and objectionable from the viewpoint of energy saving. Furthermore, the conventional green sheet laminating technique is defective because of a poor reproducibility and accordingly low yield.
The high melting point conductor material had a poor electrical conductivity. Each conductor for the wirings must therefore be appreciably thick. This results in considerably large dimensions of the wirings.
As will later be described in detail in the description of preferred embodiments of the instant invention, each green sheet has a thickness of 200 microns or more insofar as the conventional green sheet laminating technique is resorted to. It is therefore impractical to laminate a sufficient number of insulator layers. The smallest possible diameter of through holes is dependent on the thickness of each green sheet. Also in this respect, it has been difficult to achieve fine or minute wirings and consequently highly densed mounting of the semiconductor chips.
It is known to make the wirings including resistors and capacitors. In a multilayer ceramic substrate manufactured by the green sheet laminating technique, the resistors and/or the capacitors are fabricated according to the thick film printing method. For this purpose, thick film paste is fired at a temperature between 700.degree. C. and 900.degree. C. in air rather than in a reducing atmosphere. This unavoidably results in oxidation of the conductor material. It is not feasible to make each capacitor have a large electrostatic capacity because each insulator layer is appreciably thick as exemplified before. The electrostatic capacity is restricted to several hundred picofarads at most.
An approach to solve the problem of oxidation, is to plate gold on exposed surfaces of the conductor material. This is expensive and has not been widely put into practice.
Another approach is to improve the thick film paste so that firing may be carried out either in a neutral atmosphere or at a low temperature. Only a small number of varieties of the improved thick film paste are known. Moreover, the improved thick film paste is still defective and can not be used in practice because the paste raises an interlayer leakage current through the insulator layer, is unreliable, and renders the substrate poorly reproducible.
According to the green sheet printing technique, the remaining one of three schemes described hereinabove, an assembly is manufactured by printing layers of an insulating material on a green sheet together with interposed wirings. The assembly is subsequently fired. This makes it possible to fabricate thin insulator layers. It is, however, impossible to make the substrate have an appreciable number of insulator layers and to make the capacitors have a large electrostatic capacity.
Among the three schemes, it appears that the green sheet laminating technique is most promissing. The problem resides in the fact that a composition for fabricating the green sheets must be sintered into insulator layers at a considerably high sintering temperature.
In an attempt to lower the sintering temperature, glass is used in either the thick film printing or the green sheet methods and fired at a low sintering temperature into ceramic insulator layers, which are in fact glass or vitreous insulator layers. This attempt has, however, not successfully been put into practice because a multilayer glass or ceramic substrate manufactured thereby becomes porous to make the substrate have a high interlayer leakage current and a low thermal conductivity. In addition, it becomes difficult to find a conductor material which optimally matches with glass. This is because a number of bubbles appear in the wirings and in those portions of the glass insulator layers which lie on the wirings.
On the other hand, in general, it is mandatory for a multilayer substrate to have a sufficient mechanical strength to allow mounting thereon of the semiconductor chips and mounting thereof on apparatus in which the semiconductor chips are put into operation. It is known that a multilayer substrate should have a flexural strength which is not less than 2,000 kg/cm.sup.2. In spite of this, known multilayer glass substrates have a flexural strength of 1,800 kg/cm.sup.2 at most.
A multilayer glass substrate manufactured of a crystalline glass composition, has a poor thermal conductivity, such as 0.002 cal/cm.sec..degree.C. or less. This lengthens the signal propagation delay of the semiconductor chips mounted thereon. Such glass substrates are therefore inadequate for densely mounting the semiconductor chips thereon.
In the meanwhile, a multilayer glass-ceramic substrate is disclosed in U.S. Pat. No. 4,301,324 issued to Ananda H. Kumar et al and assigned to International Business Machines Corporation. A glass-ceramic article is characterized according to Kumar et al by a continuous glassy network composed of (A) beta-spodumene with the interstices of the network occupied by residual glass including crystallites of lithium metasilicate or (B) alpha-cordierite with the interstices occupied by residual glass including crystallites of clinoenstatite. The glass-ceramic article is excellent for use in composing glass-ceramic insulator layers in many respects. The substrate composed of the article is nevertheless still disadvantageous as regards the flexural strength and the thermal conductivity.